[pjc50]

I suspect it's the underlying systems that aren't good at handling online decreases. But the good news is, if you're happy to lose data, you can just delete the volume and create a smaller one.

(Yes I know that's probably not what you meant, but it does highlight the question of what exactly is non-valuable data?)

[falsedan]

I'm too lazy to attach new storage, sync data, and swap their mount points in place.

  > what exactly is non-valuable data?

Caches, mirrors, backing volumes for redundant data stores or processing infrastructure that indicates to try again on another node on failure.